Among green vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles employ a motor as well as an engine as power sources to reduce exhaust gas and to improve fuel efficiency, and have a power transmission system which separately transmits power of the engine or the motor to drive wheels, or transmits power of both the engine and the motor to the drive wheels.
An exemplary power transmission system for hybrid electric vehicles includes, as exemplarily shown in FIG. 1, an engine 10 and a motor 12 disposed in series, an engine clutch 13 disposed between the engine 10 and the motor 12 to transmit or interrupt power of the engine 10, a transmission 14 to shift and transmit power of the motor 12 or both power of the motor 12 and power of the engine 10 to drive wheels and then to output the power, a hybrid starter generator (HSG) 16 which is a kind of motor connected to a crank pulley of the engine 10 so as to transmit power and generate electricity to start the engine 10 and recharge a battery, an inverter 18 to control the motor 12 and to control electricity generation, and the rechargeable high voltage battery 20 connected to the inverter 18 so as to provide electricity to the motor 12.
Such a power transmission system for hybrid electric vehicles, in which the motor 12 is mounted close to the auto transmission 14, is referred to as a transmission mounted electric device (TMED) type and provides driving modes such as an electric vehicle (EV) mode only for electric vehicles in which only power of the motor 12 is used, a hybrid electric vehicle (HEV) mode in which the engine 10 is used as a main power source and the motor 12 is used as an subsidiary power source, and a regenerative braking (RB) mode in which, when the vehicle is braked or is driven using inertia, the braking and inertia energy of the vehicle is recovered through power generation of the motor 12 and thus recharges the battery 20.
With reference to FIG. 2, if transmission input torque input from the engine 10 and/or the motor 12 is constant, transmission output torque is changed due to clutch connection or separation during a shift and, thus, a driver feels a sense of difference caused by a shift or shift shock.
Therefore, during a shifting process of a hybrid electric vehicle, a sense of difference generated at shift is minimized through optimal slip control of a clutch within a transmission and a brake using a fluid (a transmission oil), and, in order to reduce shock generated when the clutch is connected to or separated from the inside of the transmission during the shifting process, torque intervention control to momentarily reduce transmission input torque is applied, as exemplarily shown in FIG. 2.
In such torque intervention control, a subject of torque reduction to reduce transmission input torque is an engine, a motor, or both an engine and a motor.
However, if the subject of torque reduction is both the engine and the motor, torque intervention control is carried out in a situation in which battery charging is partially restricted or a situation in which motor torque control is insufficient and torque control responsiveness is lowered, as compared to the case where the subject of torque reduction is the motor, and there is a difficulty in properly distributing torque to the engine and the motor.
Further, when engine torque for torque intervention control is reduced, a change of the ignition angle of the engine to correspond to rapid torque change causes a lowering of efficiency of the engine and lowers fuel efficiency.